You have a program that's frozen, it simply stopped responding!
1. Press the Ctrl , Shift and Esc keys on your keyboard, in quick succession.
2. You will now be presented with the Windows Task Manager window. Choose the Applications tab to determine/select the program that has stopped responding (not responding). Click on the End Task button.
3. A confirmation window should appear, giving you the option to end the program immediately, by clicking on the End Now button.
Wait a few seconds and POOF! The unresponsive program should close by itself. Your computer is back to normal!
In serious cases, the above method may fail to work and you will have to forcibly turn off your computer, by pressing the POWER button on the PC case, for a few seconds.
It is always a good idea, to give the computer some time, before you open the affected program again.